1. Field of the Invention
This invention relates to a polyethylene valve and a transition valve having at least one integral metal to polyethylene end fitting and more particularly to a metal ball valve pressure formed from pipe and having at least one end fitting adaptable for direct connection to polyethylene pipe.
2. Description of the Prior Art
Prior to the development of polyethylene pipe, it was the conventional practice to utilize metal valves connected to iron and steel pipes for the underground conveyance of pressurized fluids and gases. The metal composition of the valves and pipes required that they be treated to resist corrosion and deterioration due to cathodic action. To provide corrosion resistance and cathodic protection, the metal pipes and valves are coated with insulative material.
Metal ball valves specifically used in underground distribution systems are susceptible to corrosion. Therefore, they must be installed in a manner to permit periodic visual inspection.
The requirement to protect underground metal valves and pipes from corrosion and facilitate their visual inspection substantially adds to the installation and maintenance costs of underground metal distribution systems. Notwithstanding these factors, metal gate and ball valves are extensively used in natural gas, fuel oil and industrial piping systems because of their effective sealing capabilities and reliable performance at high pressure and temperature. U.S. Pat. No. 4,665,937 is an example of a known metal ball valve suitable for these uses.
More recently with the development of polyethylene conduit, iron and steel underground conduit has been replaced with polyethylene conduit. Polyethylene conduit is substantially lighter than metal conduit, corrosion resistant and less expensive. As underground metal conduit is required to be repaired in its maintenance cycle, polyethylene conduit is being installed in its place. Because polyethylene conduit is exempt from periodic visual inspections, it is particularly suited for use in buried natural gas distribution.
Not only are iron and steel underground conduits being replaced with polyethylene conduits, metal ball and plug valves are being replaced by equivalent valves fabricated of polyethylene. Examples of polyethylene control valves are disclosed in U.S. Pat. Nos. 3,656,711; 4,523,740; and 4,553,562. The ends of polyethylene valves are conventionally connected to polyethylene conduit by butt fusion or by electrofusion coupling.
Valves fabricated of thermoplastic material are composed of parts which are injection molded. Because of the size and expense of injection molding equipment, it has not been feasible to fabricate polyethylene ball valves having end connections greater than 4 inches without a severe reduction in internal size and flow capacity. This substantially limits the use of thermoplastic valves in natural gas distribution systems where it is preferred to use conduit having a diameter of 6 inches and larger.
Polyethylene conduit is limited in use by its pressure rating as is the polyethylene valve connected to polyethylene conduit. For example, a polyethylene ball valve connected to 4 inch diameter polyethylene conduit is limited to 100 psi for natural gas distribution. In order to use polyethylene conduit having a pressure rating greater than 100 psi, higher pressure rated polyethylene valves are required. Presently polyethylene valves of this type are not commercially available.
Thermoplastic valves rated, for example, to 275 psi are not commercially feasible because of their expense of manufacture. This is partly due to the substantial capital costs required for the injection molding equipment needed to fabricate high pressure polyethylene valves, especially for conduit 6 inches in diameter and larger. In addition, as the size and pressure rating of a thermoplastic valve increases, the difficulty in operation and sealing of the valve increases.
For a larger diameter conduit, i.e. above 4 inches, the fluid pressure on the valve body and valve closure member increases. The thermoplastic material comprising the valve body and closure member has a tendency to creep or migrate when subjected to high pressure. Creeping of the polyethylene valve body from the valve closure element can produce a gap around the closure member resulting in leakage of the valve in a closed position. In order to overcome the problem of creep or migration of the thermoplastic material in the valve at high pressure, the molded parts of the valve, including the valve seat and closure member, must be reinforced. This adds to the cost of manufacturing a polyethylene ball valve.
In view of the limitations in the use of polyethylene ball valves in conduit systems above 4 inches in diameter attempts have been made to combine metal ball and plug valves with polyethylene conduit. The combination of a metal valve and polyethylene conduit permits the practical use of larger sized polyethylene conduit and the performance capabilities of a metal valve.
One of the problems encountered in combining metal valves and polyethylene conduit is coupling the members to form a fluid tight seal. The metal pipe ends must be sealingly engaged to the polyethylene pipe. A conventional metal ball valve is formed of components which are cast and mechanically connected by nuts and bolts. The valve ends must also be mechanically coupled to the ends of the polyethylene pipe.
Many embodiments have been proposed for the compression fitting of metal conduit to polyethylene conduit. Examples of transition fittings between metal and polyethylene conduits are disclosed in U.S. Pat. Nos. 2,933,428; 3,025,086; 4,005,880; 4,682,797; 4,712,809; 4,997,214; and 5,265,652.
While the use of a steel foundation for metal ball valve eliminates many of the problems inherent with polyethylene valves, the transition between the metal and polyethylene must be effectively sealed. The above patents disclose a number of approaches to sealing the conduit ends. For larger sized conduits, the complexity and expense of the couplings increases. A fluid tight seal can be achieved, but the transition requires a number of fastening components. Accordingly, as the number of components increases, the problems of maintaining an effective seal between the components becomes more difficult. These concerns necessitate periodic visual inspection of the underground system and the metal valve must be continually monitored for the effects of corrosion.
While it has been proposed to combine metal ball and plug valves with polyethylene conduit above 6 inches in diameter for use in natural gas distribution systems, the known couplings are expensive, bulky, and difficult to install. When combined with a metal valve, periodic visual inspection of the system is required. Therefore, there is need in underground fluid distribution systems for a metal valve that is efficiently connected to both metal and polyethylene conduit or to a combination of both polyethylene and metal conduit to form a fluid tight seal at the transition and is substantially maintenance free.